This invention relates to a magnetic recording medium and a method for producing the same. More particularly, the present invention is concerned with a magnetic recording medium suitable particularly for use in applications, such as magnetic disk devices, which is excellent in wear resistance as well as in durability and reliability, and a method for producing the same.
The magnetic recording medium for use in magnetic devices etc. generally comprises a substrate which may be of various types and a magnetic layer formed thereon. The surface of the magnetic recording medium often suffers a high-rate abrasion by a magnetic head or the like during recording and reproduction of information. The lowering in the performance of the magnetic recording medium due to the above-mentioned abrasion is now a serious problem.
Therefore, it is a common practice in the art to apply a lubricant to the surface of a magnetic layer for the purpose of protecting the magnetic layer from damages due to the abrasion with a magnetic head, etc. A preferable example of such a lubricant known in the art includes a fluorinated oil, such as perfluoropolyether, as disclosed in U.S. Pat. No. 3,778,308. A currently important task is to provide a method for suitably utilizing this type of substance having a lubricity as a lubricant for a magnetic recording medium.
When the above-mentioned lubricant is applied to a magnetic recording medium, such as a magnetic disk, the greater the amount of the lubricant, the more improved the durability of the magnetic recording medium. However, excessive application of the lubricant to the surface of the magnetic layer causes an increase in the adhesion force between the head and the surface of the magnetic recording medium when the magnetic disk device is stopped, which raises problems that the head or the surface of the magnetic recording medium is damaged when the device is restarted or, in an extreme case, the device cannot be driven at all. As mentioned above, the adhesion force between the head and the medium is affected by the amount of the lubricant which is present on the surface of the medium. Hitherto, various proposals have been made with respect to a method for properly controlling the amount of the lubricant applied onto the surface of the magnetic layer which comprises rendering the magnetic layer porous and impregnating the porous layer with a lubricant.
Examples of such proposals include a magnetic disk comprising a porous magnetic layer composed of magnetic particles, a thermosetting resin of epoxyphenol resin system and non-magnetic, inorganic particles, which layer was impregnated with a lubricant (see Japanese Patent Laid-Open No. 104202/1978) and a method of preparing a porous magnetic layer which comprises adding an organic additive, such as liquid paraffin, to a magnetic paint and thermally decomposing and evaporating the additive when a coating film formed is cured (see Japanese Patent Laid-Open Nos. 3435/1981 and 10419/1985).
Further, in recent years, a proposal has been made on a high-density magnetic recording medium comprising a thermoplastic resin suitable for use in floppy disks etc., wherein an undercoating layer containing a lubricant is provided (see Japanese Patent Laid-Open No. 73235/1986).
Among the above-mentioned prior art, the method which comprises rendering a magnetic layer porous and impregnating the porous layer with a lubricant is free from a problem with respect to the lack of the lubricant, because in the conventional magnetic recording medium having a low recording density and a thick magnetic layer, it is possible to impregnate the magnetic layer with a sufficient amount of a lubricant. However, with respect to a magnetic recording medium having a high recording density for which there is an ever-increasing demand on a decrease in the thickness of the magnetic layer, it is necessary to increase the porosity of the magnetic layer in order to impregnate a thin magnetic layer with a sufficient amount of a lubricant. This leads to a problem with respect to the durability of the magnetic recording medium, such as a remarkable lowering in the wear resistance of the magnetic layer.
On the other hand, the above-mentioned prior art method which comprises adding non-magnetic particles to a magnetic coating film raises problems such as an increase in the noise output due to the additive. Moreover, the above-mentioned prior art method which comprises making a porous structure through thermal decomposition of an organic additive is disadvantageous in that a failure to choose a proper additive having a desirable compatibility with the binder of the paint leads to the formation of large pores in the coating film, thus causing noises and errors.
Furthermore, among the above-mentioned prior art methods, the method in which a lubricant is incorporated in an undercoating layer provides a structure suited for a high-density magnetic recording medium having a thin magnetic layer. However, in this method, the positions of the fine pores of the undercoating layer do not- always coincide with those of the magnetic layer and, therefore, the fine pores of the undercoating layer do not get through to the surface of the magnetic layer, which makes it impossible to impregnate the undercoating layer with a sufficient amount of a lubricant. Particularly, with respect to a magnetic disk comprising a magnetic layer composed of a thermosetting resin suited for a rigid disk, it is very difficult to impregnate the undercoating layer with a lubricant, because the thermosetting resin of the magnetic layer is very closely hardened.